Target Acquisition with Phoenix

To minimize fringing, the order sorting filters in Phoenix are wedged
slightly. Consequently, the collimated beam is deflected as it passes
through the filter. To ensure that the collimated beam is parallel to
the optical axis after passing through the filter, light entering the
spectrograph must be off-axis slightly. The required angular offset is
different for each filter because of small differences in wedge angle.
The wedges are oriented to deflect light primarily north-south, so that
reflection ghosts will not pass through the east-west slit.

A dichroic inside Phoenix is used to separate visible light from the
infrared beam headed for the filter and slit. The visible light image
is used for target acquisition and guiding. Because the dichroic comes
before the filter, location of a star on the guide camera TV depends
only on telescope orientation and not on which filter is in use. So
each filter deflects the incoming beam by a different amount, but the
deflection is not registered on the guide camera. In practice this means
that for each filter, targets must be placed in a different place on
the guide camera TV. Otherwise, the source will not be imaged onto the
slit.

There are two main techniques for locating the correct position on
the guide camera TV for a given filter. One can move the viewer to
slitviewer mode and take an IR image of the thermal background viewed
through the slit. Next, move the slit to the open position and adjust
the telescope until the source position in the IR image coincides with
the previously noted slit location. Return the slit and viewer wheels
to their original positions and take spectra. Alternatively, one can
step the telescope north-south, until a short exposure of a bright source produces at least a faint
spectrum. Refine the north-south position of the telescope to maximize
counts in the spectrum.

When using slitviewer mode at the 4-m, the image scale is 0.138 arcsec
per pixel. Moving the telescope east makes the source appear at larger
column numbers. Moving the telescope north makes the source appear at
larger line number. At the 2-m, there are approximately 1.8 times as
many arcsec per pixel. Finally, the relative offsets between several
of the available filters are given in the table below. These numbers
may be used to reposition the target approximately, when switching
between filters. Only the north-south offset is important, since
east-west displacements simply move the target along the slit.